To keep teams up to date on new building methods, the Concrete Canoe National Competition (CCNC) is constantly updating and changing the required materials for building a concrete canoe. For 2017, the CCNC required the teams to use an aggregate, besides man-made glass beads or cynospheres, which passes ASTM C330. This aggregate was required to make up at least 25% by volume of the total aggregate used. The University of North Carolina at Charlotte’s (UNCC) 2017 concrete canoe team used this new requirement to their advantage. By using expanded shale, clay and slate (ESCS) lightweight aggregate, the team was able to use the benefits of internal curing to create a concrete just as strong as the previous year, while using less cement. The aggregate was simply soaked prior to the introduction of cement.
Cement is the heaviest ingredient in concrete. To build a lightweight canoe, a team must create a strong, but very light mix. If improperly cured, concrete would not reach its full potential. By relying on only water and the absorption ability of lightweight aggregate, UNCC was able to decrease the unit weight of its concrete by reducing the amount of cement used. The strength was still maintained because of proper hydration of the concrete from inside out, thanks to the slow release of the water absorbed by the lightweight aggregate.
Due to the limited available time to produce a final mix, internal curing tests were limited to 14-day cure times. Two separate techniques were used when testing the internal curing capabilities of the lightweight aggregate. All ingredients were the same for all tests. The only difference was the time allowed for the lightweight aggregate to absorb the water prior to the introduction of the cementitious material. The amount of water absorbed by the lightweight aggregate was added in each mix in order to match workability/slump. The first technique mixed the lightweight aggregate and batch water together. They were allowed to sit in the mixer for lightweight aggregate 10 minutes prior to the introduction of the cementitious material. For the second technique, the lightweight aggregate and batch water were mixed, and placed in a 5 gallon bucket with the lid on for 24 hours. Control samples were set by mixing the dry aggregates and all the other ingredients simultaneously.
The lightweight concrete that incorporated the 10-minute wait time had 18% more compressive strength than the control. The aggregates allowed to sit for 24 hours had 37% more compressive strength than the control. In the concrete canoe competition, the weight of the canoe could make or break a team during the races. The team decided that it was worth the extra effort for the 37% strength increase and buckets of soaking aggregates were prepped the day before the canoe was constructed. The team used their internal curing technique as well as many other innovative design processes to place 3rd overall in 2017 the ASCE Carolinas Competition. Internal curing will remain as a standard mixing technique until the CCNC decides to change the rules again.
As a follow-up, the UNCC Concrete Canoe team did continue to use lightweight aggregate for internal curing in their canoe and the team placed first in the 2018 ASCE Carolinas Competition. The team and internally cured canoe competed in the ASCE National Canoe Competition in June 2018 in San Diego.